1. Field of the Invention
This invention relates generally to semiconductor devices having molded housings. More particularly, the invention pertains to a mold for encapsulating integrated circuits (ICs) which include arrays of conductive balls, pins, or pads on substrates.
2. State of the Art
Modern packaged integrated circuits (ICs) comprise one or more encapsulated semiconductor dies or chips within a protective xe2x80x9cpackagexe2x80x9d of plastic or other moldable material. The integrated circuit chips are made from a semiconductor material such as silicon, germanium or gallium arsenide, and microscopic circuits are formed on a surface of each chip by photolithographic techniques. A plurality of external connections, typically designed for soldering, is connected to bond pads on one or more encapsulated chips, enabling the chips to be electrically interconnected to an external electrical apparatus. In one form of interconnection, a substrate such as a wiring board or circuit board has an array of conductors which is typically connected to the wire bond pads of the chips and which extends through the substrate to the opposite side for conductive, e.g. solder, connection to an electronic apparatus. In addition to one or more chips attached to the substrate in either packaged or unpackaged form, other devices such as resisters, capacitors, etc. may be mounted to the substrate and incorporated in the circuit.
Plastic encapsulation of semiconductor and other electronic devices by transfer molding is a well-known and much-used technique. Typically, a large number of components or devices is placed in a lower mold plate or half of an open multi-cavity mold, with one or more devices within each cavity. The mold is closed with a mating upper plate. The cavities of the mold are connected by xe2x80x9crunnersxe2x80x9d, i.e. channels to a xe2x80x9cpotxe2x80x9d or reservoir from which pressurized liquified plastic is fed. Typically, constricted channels known as xe2x80x9cgatesxe2x80x9d are located at the entrance to each mold cavity to limit the flow rate and injection velocity of liquified plastic into the cavity.
Where it is desired to encapsulate a circuit board or wiring board upon which one or more semiconductor devices have been mounted and wired, a peripheral portion of the board (or of a portion encompassing a mounted circuit) is compressed between the upper and lower plates to prevent leakage of liquified plastic from the mold cavity. Typically, the force required to compress the platen together is of the order of tons, even for molding machines having only a few mold cavities.
Typically, powdered or pelletized plastic, e.g. thermoset resin, is placed in the resin pot and compressed by a ram. The heated, pressurized plastic becomes liquified and flows through the runners and gates to surround each device and fill each mold cavity, where it subsequently hardens to encapsulate the board and the devices attached to it. Air is expelled from each cavity through one or more runners as the plastic melt fills the mold cavities. Following hardening by partial cure of the thermoset plastic, the mold plates are separated along the parting line and the encapsulated devices are removed and trimmed of excess plastic which has solidified in the runners and gates. Additional thermal treatment completes curing of the plastic package.
Following removal of each encapsulated unit from its mold cavity and curing, the peripheral portions of the board may be excised from the board and any flash is removed as known in the art, and the device is ready for use.
In many devices having a ball grid array (BGA) or similar array on a circuit board, the molding process is conducted so that the surface of the circuit board having the array connections forms an outer surface of the package, not being covered by the plastic material. Thus, any plastic which has solidified on the array connection surface is removed.
In most devices, the substrate material which is clamped between the platens may be variable in thickness or density from lot to lot, or may actually change in thickness during the molding process due to compression and/or release of gases therefrom. It is known, of course, that the high compression forces will typically compress a circuit board significantly. The variation in thickness at a given compression force may be xc2x15 mils or more. Unless there is compensation for this variability, a thinner-than-normal circuit board may result in leakage of encapsulant plastic past the mating portions of the upper and lower mold plates.
In U.S. Pat. No. 5,609,889 of Weber, a mold is described which has a biased plug that exerts pressure on a heat sink or circuit board to prevent molding compound from covering the heat sink or circuit board. A passage is provided in the substrate circuit board so that plastic flows latitudinally under the circuit board into a cavity. The plug is biased by a plate spring to accommodate variations in the thickness of the substrate and ensure that the exterior surface of the heat sink does not become significantly encased in plastic. The pressure exerted by the plate spring need only be nominal, enough to hold the circuit board or heat sink against the xe2x80x9cfloorxe2x80x9d of the mold cavity. The circuit board is not clamped between the plate walls, so any variation in thickness of the board will not cause leakage of plastic past the mating plate surfaces.
It has been discovered that gases are commonly released from substrates such as circuit boards during transfer molding, presumably because of substrate compression as well as degasification due to the elevated molding temperatures. Mold plates which clamp a substrate about a mounted IC device are completely sealed on the side of the substrate which is not to be encapsulated. The buildup of gases (and pressure) on one side of the substrate may cause temporary warping of the substrate. Upon removal from the mold plates, the residual forces in the substrate may cause cracking, spalling, etc. in the package.
The present invention provides a molding machine for providing one-side encapsulation of electronic devices having a substrate such as a circuit board upon which are mounted integrated circuits (ICs) optionally with other apparatus such as resistors and capacitors.
The present invention also provides a molding machine having means to compensate for batch-to-batch variations in substrate thickness. The present invention provides substrate thickness compensation which automatically adjusts for variations in substrate thickness about a nominal thickness, and which is easily adjustable for different nominal substrate thicknesses. The present invention provides a molding apparatus having substrate thickness compensation which is readily usable with existing commercial molding machines.
The present invention further provides apparatus for venting gases released from a substrate during one-side molding encapsulation of an electronic apparatus mounted on the substrate.
The present invention further provides apparatus which is easily constructed from existing components at low cost.
The invention comprises improvements in a molding machine for encapsulating electronic devices mounted on the first side of a substrate such as a circuit board or wiring board, as exemplified by a board having a ball-grid-array (BGA), pin-grid-array (PGA), land-grid-array (LGA) or similar set of multiple electrical terminals on its opposite side. Such a circuit board is configured to have the array terminals bonded to another apparatus following encapsulation of the electronic devices including chip(s), wiring and other devices on its first side.
One aspect of the invention comprises a biased floating plate apparatus which comprises a floating plate, which may be a mold plate or a platen, and a second member attached to the floating plate. The second member may be a mold chase, for example. A biasing means associated with the second member biases the combination toward a first thickness dimension t, at which no outward force is exerted by the floating plate apparatus. A compressive force P is required to diminish the thickness t of the apparatus, and the inverse relationship between the compressive force P and thickness t is readily adjustable over a desired working range. The biased floating plate apparatus is positioned within the compression train of the molding machine to provide the desired compensation for inherent or process-caused variations in substrate thickness. The floating plate is biased by a plurality of springs or other force-compressible means or suitable resilient means spacedly mounted in the second member, a mold chase, for example.
Another aspect of the invention comprises a relief area and vent within the cavity, on the lower mold plate, covering all or a portion of the xe2x80x9cfloorxe2x80x9d of each mold cavity. The relief area and vent serve to collect and discharge gases emitted by the substrate. The emission of such gases is due to compression at the clamped portions of the substrate, and the increased temperature during encapsulation. Unless relieved, the pressure from such gases may warp the substrate during encapsulation. Return of the substrate to its original planar position after removal from the mold may lead to cracking and spalling of the encapsulant from the substrate. Use of the relief area and vent of the invention results in a stable electronic package.
The combination of the biased floating plate apparatus and plate relief/vent provides a molding machine with significant advantages in production methodology, including an increase in production up-time and cost-effectiveness.